This invention generally relates to flow control valves and more particularly to a brake valve for a hydraulically driven winch used to control the flow of hydraulic fluid out of the winch motor when it is being used to lower a load. This valve is particularly suited for use with a hydraulically driven high speed winch similar to those currently used on mobile cranes. It is designed to act in conjunction with the hydraulic winch motor, winch brake, overrunning clutch and gear train to provide precise load control under varying load conditions and hydraulic oil temperatures.
Brake valves are a special type of flow control valve and are generally well known in the art. In the past, these valves were typically pilot operated poppet type counter balance valves in which the poppet had a tapered end which fitted into a mating tapered seat. A pilot circuit was used to transmit pressure from the motor inlet to a pilot piston connected to the poppet. One undesirable characteristic of such valves was that a relatively small movement of the poppet would produce a relatively large variation in oil flow rate through the valves, resulting in stability problems during load lowering operations. In more recent brake valves the poppet has been replaced with a spool and the flow is required to pass through more than one orifice to reduce the sensitivity of the valve to pressure fluctuations in the motor control circuit.
In spite of these improvements, stability problems with such valves still persist. Typically, these problems arise when the winch control valve is moved too quickly into a full open position to lower a load and are particularly noticeable when the load is light. Upon a sudden movement of the control valve, the winch brake is released, fluid pressure rises rapidly in the pilot circuit, and the control spool is moved very quickly into a full open position. In this position the spool permits a sudden surge of fluid to pass through the motor, accelerating it to maximum r.p.m., causing the cable to go slack and permitting the load to fall free. When the motor is momentarily unloaded in this manner while running at high speed, fluid pressure at the motor inlet and in the pilot circuit drops quickly, causing the spool to close and slow the motor down abruptly. When the slack is gone from the cable, the fall of the load is arrested, often violently, and the tension on the cable increases suddenly. Subsequently, motor inlet and pilot pressures rise again, and this entire sequence of events is repeated. Often, the only way the operator can bring the system under control is to stop the winch. It has also been found that stability problems of this type are more likely to occur when the hydraulic oil is hot.
Accordingly, it is a primary object of this invention to provide for an improved brake valve for hydraulically driven winches which will exhibit better stability characteristics and less sensitivity to fluid operating temperatures during load lowering operations than those valves currently known in the art.